run unshare --mount with --propagation unchanged to prevent 'cannot change root filesystem propagation' when running mmdebstrap from inside a chroot

coverage.sh

@@ -119,7 +119,7 @@ if [ ! -e shared/hooks/eatmydata/customize.sh ] || [ hooks/eatmydata/customize.s
 	fi
 fi
 starttime=
-total=189
+total=190
 skipped=0
 runtests=0
 i=1
@@ -518,6 +518,43 @@ else
 	runtests=$((runtests+1))
 fi
 
+# Before running unshare mode as root, we run "unshare --mount" but that fails
+# if mmdebstrap itself is executed from within a chroot:
+# unshare: cannot change root filesystem propagation: Invalid argument
+# This test tests the workaround in mmdebstrap using --propagation unchanged
+print_header "mode=root,variant=apt: unshare as root user inside chroot"
+cat << END > shared/test.sh
+#!/bin/sh
+set -eu
+export LC_ALL=C.UTF-8
+[ "\$(whoami)" = "root" ]
+cat << 'SCRIPT' > script.sh
+#!/bin/sh
+set -eu
+rootfs="\$1"
+mkdir -p "\$rootfs/mnt"
+[ -e /usr/bin/mmdebstrap ] && cp -aT /usr/bin/mmdebstrap "\$rootfs/usr/bin/mmdebstrap"
+[ -e ./mmdebstrap ] && cp -aT ./mmdebstrap "\$rootfs/mnt/mmdebstrap"
+chroot "\$rootfs" env --chdir=/mnt \
+	$CMD --mode=unshare --variant=apt \
+	$DEFAULT_DIST /tmp/debian-chroot.tar $mirror
+SCRIPT
+chmod +x script.sh
+$CMD --mode=root --variant=apt --include=perl,mount \
+	--customize-hook=./script.sh \
+	--customize-hook="download /tmp/debian-chroot.tar /tmp/debian-chroot.tar" \
+	$DEFAULT_DIST /dev/null $mirror
+tar -tf /tmp/debian-chroot.tar | sort | diff -u tar1.txt -
+rm /tmp/debian-chroot.tar script.sh
+END
+if [ "$HAVE_QEMU" = "yes" ]; then
+	./run_qemu.sh
+	runtests=$((runtests+1))
+else
+	./run_null.sh SUDO
+	runtests=$((runtests+1))
+fi
+
 print_header "mode=root,variant=apt: fail with root without cap_sys_admin"
 cat << END > shared/test.sh
 #!/bin/sh

mmdebstrap

@@ -4482,8 +4482,13 @@ sub main() {
         # For unshare mode to work we either need to already be the root user
         # and then we do not have to unshare the user namespace anymore but we
         # need to be able to unshare the mount namespace...
+        #
+        # We need to call unshare with "--propagation unchanged" or otherwise
+        # we get 'cannot change root filesystem propagation' when running
+        # mmdebstrap inside a chroot for which the root of the chroot is not
+        # its own mount point.
         if ($EFFECTIVE_USER_ID == 0
-            && 0 != system 'unshare --mount true 2>/dev/null') {
+            && 0 != system 'unshare --mount --propagation unchanged -- true') {
             error "unable to unshare the mount namespace";
         }
         # ...or we are not root and then we need to be able to unshare the user